1. Field of the Invention
The present invention relates to a method for generating a calibration curve, and more particularly to a method for generating a calibration curve by selecting reference points of one image line instead of all points.
2. Description of the Related Art
Optical scanners are used to capture and digitize images. For example, an optical scanner can be used to capture the image of printed matter on a sheet of paper. The digitized image can then be electronically stored and/or processed with character recognition software to produce ASCII text. The typical optical scanner includes a light source, a linear array of photoelectric sensing elements (generally a CCD sensor or a CMOS sensor, or a CIS sensor), an analog amplifier, an analog to digital converter (ADC), a controller and a random access memory (RAM).
The CCD sensor includes a large number (e.g., 2000) of photoelectric sensing elements arranged in a linear array. Each photoelectric sensing element will capture light representing a single pixel of the image. The array will capture a line of pixels. By moving the CCD sensor across a document, the entire document can be scanned one line at a time.
The conversion into digital signals of light reflected from or transmitted through the document takes place in essentially three steps. First, each photoelectric sensing element will convert the light which it receives into an electric charge. The magnitude of the charge will depend on the intensity of the light and the exposure time. Second, the charges from each of the photoelectric sensing elements are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized by the analog to digital converter for digital image processing and storage in the RAM.
In a conventional image scanner, calibration is necessarily performed before scanning images. This is because the conditions of the image components of an image scanner usually alter in every scanning. For example, the brightness of the light source comprising a cold-cathode fluorescent lamp (CCFL) usually varies with duration of turn-on, environment temperature, and even its location. Moreover, linear image sensors such as CCD will not be identical and always respond differently to the same light signal. A total white calibration board or a full black calibration board is conventionally used to generate a shading curve or calibration curve. By scanning the white calibration board or full black calibration board, a user can obtain a calibration curve showing how the bright level of the generated image varies with pixel positions of one image line. The data of this calibration curve are then used to calculate or to generate calibration data corresponding to each image sensor or CCD. However, the above mentioned calibration method has some troubling drawbacks. For example, a linear image array may have many sensors or may be 10,000 sensors for each of red, green and blue (RGB) colors constituting one image line. If 32 image lines are chosen to generate calibration curves, then there are 10,000 (pixels)×3 (RGB)×32 (lines)=960,000 pixels to be processed. Each pixel comprises 1 to 2 bytes of data and each byte represents a level value from 0 to 255. It is apparent that such a huge data quantity to be processed would drag the speed and performance of an image sensor. Some conventional image scanners skip generating a calibration curve before every scanning and utilize the previous calibration curve to generate calibration data. However, the previous calibration curve always differs from the present calibration curve since the conditions of the components of an image scanner usually alter in every scanning.
Hence it is desirable to provide a method for generating a calibration curve so as to render an operator or a manufacturer of an image scanner easier and more effective to calibrate images generated by the image scanner.